Infrared Spectroscopy
• Bonds within molecules vibrate at all temperatures above 0 kelvin.
• Each bond vibrates at a characteristic frequency within the infrared region of the electromagnetic spectrum.
• If infrared light is passed through a sample, it will absorb light at the frequencies at which its bonds vibrate.
• Note: Not all bonds absorb energy at infrared wavelengths. For energy to be absorbed, the vibration of a bond
needs to cause a change in the dipole of the overall molecule.
Infrared Spectrometer
• An infrared spectrometer produces a graph that shows you what frequencies of radiation the molecules are
absorbing, so you can use it to identify functional groups.
• The graph plots wavenumber (cm-1) versus transmission (%).
o Wavenumber = 1/Wavelength.
• Most peaks are sharp but those containing hydrogen bonds can be broad.
The Fingerprint Region
• The region between 1500cm-1 and 400cm-1 is unique to a particular compound.
• You can use a computer database to check this region of an unknown compound’s IR spectrum against those
of known compounds.
• Signals above 1500cm-1 can be used to identify functional groups.
Global Warming
• When the bonds in greenhouse gases absorb IR radiation, they vibrate more, and so this radiation does not
escape the atmosphere.
• A general increase in the kinetic energy of molecules in the atmosphere is detected as temperature increases
→ global warming.
1. The sun emits light, which is absorbed and re-emitted by the earth (NOT reflected) – because the
earth is cooler, this is at lower energy IR wavelengths.
2. Greenhouse gases absorb IR light. Some of this is re-emitted back towards to the earth’s surface
rather than escaping to the atmosphere.
3. The trapped IR energy causes the earth to be heated.
Mass Spectrometry
• Mass spectrometry can be used to find the relative molecular mass (Mr) of a compound.
• In the mass spectrometer, a molecular ion (M+) is formed when a molecule loses an electron.
• The molecular ion produces a molecular ion peak on the mass spectrum of the compound.
• For any compound, the mass/charge (m/z) value of the molecular ion peak will be the same as the molecular
mass of the compound.
High Resolution Mass Spectrometry
• High resolution mass spectrometers can measure atomic and molecular masses extremely accurately – to
several decimal places.
• This can be useful for identifying compounds that appear to have the same Mr when they’re rounded to the
nearest whole number.
o For example, both C3H8O and C2H4O2 have a Mr of 60 (rounded to the nearest whole number).
o However a high resolution mass spectrometer will measure the Mr of C3H8O as 60.0575 and the Mr of
C2H4O2 as 60.0210.
• Note: High resolution mass spectrometry gives the molecular formula, but not necessarily the compound due
to the presence of isomers.
o For example, if the Mr is 60.0575 then the molecular formula is C3H8O, but the compound could be
propan-1-ol, propan-2-ol or methyoxyethane.
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• Bonds within molecules vibrate at all temperatures above 0 kelvin.
• Each bond vibrates at a characteristic frequency within the infrared region of the electromagnetic spectrum.
• If infrared light is passed through a sample, it will absorb light at the frequencies at which its bonds vibrate.
• Note: Not all bonds absorb energy at infrared wavelengths. For energy to be absorbed, the vibration of a bond
needs to cause a change in the dipole of the overall molecule.
Infrared Spectrometer
• An infrared spectrometer produces a graph that shows you what frequencies of radiation the molecules are
absorbing, so you can use it to identify functional groups.
• The graph plots wavenumber (cm-1) versus transmission (%).
o Wavenumber = 1/Wavelength.
• Most peaks are sharp but those containing hydrogen bonds can be broad.
The Fingerprint Region
• The region between 1500cm-1 and 400cm-1 is unique to a particular compound.
• You can use a computer database to check this region of an unknown compound’s IR spectrum against those
of known compounds.
• Signals above 1500cm-1 can be used to identify functional groups.
Global Warming
• When the bonds in greenhouse gases absorb IR radiation, they vibrate more, and so this radiation does not
escape the atmosphere.
• A general increase in the kinetic energy of molecules in the atmosphere is detected as temperature increases
→ global warming.
1. The sun emits light, which is absorbed and re-emitted by the earth (NOT reflected) – because the
earth is cooler, this is at lower energy IR wavelengths.
2. Greenhouse gases absorb IR light. Some of this is re-emitted back towards to the earth’s surface
rather than escaping to the atmosphere.
3. The trapped IR energy causes the earth to be heated.
Mass Spectrometry
• Mass spectrometry can be used to find the relative molecular mass (Mr) of a compound.
• In the mass spectrometer, a molecular ion (M+) is formed when a molecule loses an electron.
• The molecular ion produces a molecular ion peak on the mass spectrum of the compound.
• For any compound, the mass/charge (m/z) value of the molecular ion peak will be the same as the molecular
mass of the compound.
High Resolution Mass Spectrometry
• High resolution mass spectrometers can measure atomic and molecular masses extremely accurately – to
several decimal places.
• This can be useful for identifying compounds that appear to have the same Mr when they’re rounded to the
nearest whole number.
o For example, both C3H8O and C2H4O2 have a Mr of 60 (rounded to the nearest whole number).
o However a high resolution mass spectrometer will measure the Mr of C3H8O as 60.0575 and the Mr of
C2H4O2 as 60.0210.
• Note: High resolution mass spectrometry gives the molecular formula, but not necessarily the compound due
to the presence of isomers.
o For example, if the Mr is 60.0575 then the molecular formula is C3H8O, but the compound could be
propan-1-ol, propan-2-ol or methyoxyethane.
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